PHYLOGENETIC 39 



reduction of the toes to four, and the lengthening and approxi- 

 mation of the metatarsals, 2-4, to form a " cannon-bone ". 



The body clothing 

 at this time was prob- 

 ably scaly, but with 

 scales of relatively large 

 size. Those covering the 

 hinder border of the in- 

 cipient wing, growing 

 longer, would still retain 

 their original overlapping 

 arrangement, and along 

 its hinder border would, 



III. 12. — One of the Pro-Aves 



m their arrangement, ap- 

 pearance, and function, simulate the quills of modern birds ; 

 as their length increased they became also fimbriated and more 

 and more efficient in the work of carrying the body through 

 space. 



There is less of imagination than might be supposed in this 

 attempt at reconstructing the primitive feather, inasmuch as 

 there is a stage in the development of the highly complex 

 feather of to-day which may well represent the first stage in 

 this process of evolution. 



Creatures such as are here conjured up would bear a 

 somewhat close resemblance to the Archaeopteryx ; and it is 

 contended that the discovery of earlier phases of avian de- 

 velopment, the phases preceding Archaeopteryx, will show that 

 this forecast was well founded. But in Archaeopteryx it is to 

 be noted the feathers, so far as the impressions on the slab 

 containing the remains of this bird enable us to see, differ in no 

 way from the most perfectly developed feathers known to us. 



While the external form and mode of life of these primitive, 

 hypothetical types was slowly changing, no less fundamental 

 changes must have been taking place with regard to the inter- 

 nal organs, more especially to the nervous, respiratory and 

 vascular systems, changes in the direction of a larger brain in 

 the one, and a more perfect system of oxygenating the blood 

 in the other. This last was effected by the acquisition of a 

 four-chambered heart, an approach to which has been made only 

 in the Crocodiles among living reptiles. By the addition of this 



